The present invention relates generally to the fired clay art, and more particularly to a novel, high-strength, light-weight fired clay body such as a roof tile, and to a method for producing it.
As is generally known, the fired clay art is an extremely ancient one, and the production of fired clay roof tile extends back many years.
At the present time, fired clay bodies such as roof tile are produced primarily in a stack-type kiln, with the pressed unfired or green clay bodies being dried for from several hours to several days, and fired with a relatively low heat for as long as eight hours. This slow and costly procedure produces a completely oxidized or matured tile which has a relatively low strength to weight ratio, which tends to warp during firing, and which shrinks as much as ten percent, or even more, during the drying and firing steps. The term "maturity" is used in the fired clay art to describe a surface which is oxidized to the ultimate hardness.
The so-called "walk-on" strength of fired roof tile is usually determined by supporting the tile at opposed edges and applying a transverse force at the center of he tile until it breaks. Large roof tiles, e.g. nine inches by 14 inches, produced by the aforementioned slow-fired process, weigh in the neighborhood of 6 to 8 pounds and usually break under loads of 200 pounds or less so as to have a strength to weight ratio of about 25-33. In short, it is difficult to produce roof tile following known methods, which will satisfy the "walk-on" strength requirements of various governmental agencies, and yet not be too heavy.
It is recognized that high strength tile can be produced utilizing known methods, provided special high-strength materials such as Petalite are used in the tile body or the tile is made unusually thick, but such tiles are not practical from an economical standpoint.
An additional problem in the production of roof tiles, which are usually formed by the extrusion method, is the production of tiles which have lugs at the edges thereof.
Another problem in the production of relatively large roof tiles by pressing, as contrasted with extruding, is the clinging of the green or unfired material to the faces of the forming die, thereby requiring the presses to be stopped at frequent intervals to remove the unpacked material and clean the faces of the dies.